This invention relates to a glazing panel comprising clear glass panes held in spaced facing relationship and incorporating a coating on an exterior glass surface of the panel for solar shading purposes.
Such panels are known wherein one of the exterior glass faces bears a coating which is capable of screening off a proportion of incident solar radiation. When used as a window with the coated exterior face to the outside of the building the coating reduces the glare and/or the heating effect of strong sunlight at the building interior.
Such an external coating can be formed of one or more oxides. Oxide coatings can provide a useful shading effect against solar radiation while having an adequate visible light transmissivity to meet most glazing requirements. Such oxide coatings can have a fairly high abrasion resistance and they can be formed on large areas of glass with a high degree of uniformity. These potentialities of oxide coatings are well known in the art of coated glass manufacture and various oxide coatings are in actual use.
A disadvantage of such optical oxide coatings is the heating effect associated with their screening function. The solar shading afforded by such coatings is appreciably dependent on their absorption of light and/or infra-red radiation. This energy absorption results in heating of the coated pane and the re-emission of energy as long wavelength infra-red radiation. Some of this re-emitted energy is radiated towards the interior of the panel, i.e. towards the interior of the building, and consequently detracts from the overall shading efficiency of the panel.
The adverse effects of the energy absorption by the external coating can be reduced by screening off the infra-red radiation emitted internally from the coated exterior pane, e.g. by a suitable optical coating on the next pane. It is theoretically better however to coat the inside face of the oxide-coated pane itself in order to reduce the infra-red emission from that face. In practice however there are problems in reconciling the provision of such an internal coating with required performance specifications of the panel if these specifications require its luminous transmission factor to be high having regard to its total energy transmission factor.
As used in this specification the term "luminous transmission factor" denotes a ratio of the quantity of visible transmitted light to the quantity of incident visible light, such quantities being corrected integrations of the transmitted and incident light values respectively over the whole spectral range of visible light, the integrations being corrected to compensate for the spectral distribution of the radiant energy source and for the spectral sensitivity characteristics of the human eye. The measurements are made with a spectrophotometer and using a light source whose spectral composition is that of Illuminant D 65 as defined by the International Commission on Illumination (reference CIE 17 Sections 45-15-145). This illuminant represents daylight with a colour temperature of about 6504 K. The eye sensitivity correction factor applied is likewise that which is standardised by the International Commission on Illumination.
The term "total energy transmission factor" as used herein denotes the ratio of transmitted radiant energy to incident radiant solar energy. The term "energy absorption factor" as used herein denotes the fraction of incident radiant solar energy which is absorbed. For the determination of both of these factors use is made of a radiator whose spectral composition is that of direct sunlight at an elevation of 30.degree. above the horizon. The spectral composition is given by Moon's Table for a mass of air equal to 2. The energy absorption factor of a coated glass pane as referred to in this specification, like the total energy transmission factor of a panel, is always measured with the face bearing the energy-absorbing coating directed towards the radiant energy source. The luminous transmission factor is not dependent on whether the face bearing said energy-absorbing coating is directed towards or away from the light source.